The invention relates to a method for minimizing the autocorrelation error in the demodulation of a spread-spectrum signal subject to multipath propagation, according to the precharacterizing clause of Patent claim 1, and to an arrangement for demodulating a spread-spectrum signal subject to multipath propagation, according to the precharacterizing clause of Patent claim 5.
Wire-free transmission methods based on a spread spectrum have been used for many years for data communication between transmitting or receiving stations on the earth and transmitting or receiving stations in satellites. In many cases, the direct sequence method is preferred as the spreading method, in which a low-rate digital information signal is modulated with a high-rate, pseudo-random binary sequence. If the receiver knows the pseudo-random binary sequence used for modulation in the transmitter, it can extract the original, digital information signal from the pseudo-noise signal produced in this case.
Such methods are used for data communication, position-finding and navigation. One important field of application is, for example, real-time satellite navigation using the NAVSTAR GPS (Navigation System with Timing and Ranging, Global Positioning System) which is described, for example, in SCHRxc3x96DTER, GPS Satelliten-Navigation [GPS satellite navigation], Franzis-Verlag Munich, 1994. In this case, a large number of satellites orbiting in different orbit trajectories emit a large number of different position-finding signals, of which at least a specific minimum number can be received at any point on the earth""s surface. The location of the receiver can be calculated and output in a conventional coordinate system, from the received position-finding signals.
In order that a receiver can identify a satellite transmitter and can evaluate the position-finding information transmitted by it, said receiver must know the transmitter-specific pseudo-random binary sequence (gold code) which is transmitted as a periodic signal sequence of predetermined length. In order to find the code, all the codes of the transmitters in question are stored in the receiver. Since the phase angle of each incoming signal is unknown, the match between the received spread sequence (gold code) and a locally produced spread sequence is found in an acquisition method. The comparison is carried out via the correlation function of the signal, which becomes a maximum when the received spread signal is synchronized with the locally produced spread sequence.
Since the transmitter and the receiver move relative to one another, synchronization is maintained by slaving the phase angle of the locally produced spread sequence to the received spread sequence. This is done by controlling the clock of the locally produced spread sequence as a function of the determined phase error.
Until now, a delay locked loop (DLL) has been used as the control loop for this purpose, as is described, for example, by J. K. Holmes in xe2x80x9cCoherent Spread Spectrum Systemsxe2x80x9d, Robert E. Krieger, 1990. A similar description is also given by Helmuth Lemme in the article xe2x80x9cSchnelles Spread-Spectrum-Modem auf einem Chipxe2x80x9d [fast spread-spectrum modem on a chip disk] in Elektronik [Electronics] 15/1996, pages 38 to 45, in particular in FIG. 7 there. The DLL method is based on the fact that the locally produced spread sequence is phase-shifted by the same amount before and after the expected point in time, and the received spread sequence is correlated with the spread sequences of this earlier and later time separately from one another in two correlators. The correlation values emitted from the correlators are then subtracted from one another. The control loop is set in such a manner that the subtraction result becomes zero in the steady state.
However, in a real reception environment, the receiver often does not just receive the transmitted signal on the direct path, but a portion of the received signal results from reflection of the transmitted signal on a nearby terrain profile or a nearby building. This reception situation is called multipath propagation. The receiver accordingly receives a signal mixture comprising a superimposition of the transmitted signal, with a plurality of phase angles each having a different amplitude. This on the one hand exacerbates signal acquisition in the receiver and, furthermore, corrupts the position-finding result, since the position-finding calculations are based on the signal reception times for the directly received transmitted signal.
The invention is thus based on the technical problem of specifying a method and an apparatus by means of which the phase angle of the locally produced spread sequence is slaved as accurately as possible to the directly received signal once the signal has been acquired.
The problem is solved by a method having the features of Patent claim 1. The problem is furthermore solved by an arrangement having the features of Patent claim 5. Advantageous refinements of the method and of the arrangement are specified in the respective dependent claims.
In one embodiment of the method for minimizing the autocorrelation error in the demodulation of a spread-spectrum signal subject to multipath propagation, the received spread it sequence is supplied to a first control loop and to a second control loop. The two control loops operate on the principle of a delay locked loop DLL. The first control loop operates with a first phase-angle pair, which has a first phase spacing or early-late spacing 2xcex941. The second control loop operates with a second phase-angle pair, which has a second early-late spacing 2xcex942. The two control loops are supplied with a locally produced spread sequence, for correlation with the received spread sequence. The first control loop emits a first point phase value T1 in the steady state, and the second control loop emits a second point phase value T2 in the steady state. The method is characterized in that the phase value T0 (which is supplied to a demodulation correlator) of the local spread sequence with respect to the received spread sequence is calculated using the formula T0=(xcex942T1xe2x88x922 xcex941T2)/(xcex942xe2x88x92xcex941), where xcex942 is chosen to be less than xcex941. With the use of an additional control loop, the method allows any control error contained in each individual control loop as a result of the signal being distorted by multipath propagation to be eliminated by means of a simple rule, and thus allows a calculation to be made back to the correct time of the directly received signal. The rule can advantageously be used continuously and provides correct results without the receiver having to detect whether multipath propagation is or is not present.
In one preferred refinement of the method, the ratio of the first early-late spacing 2xcex941 to the second early-late spacing 2xcex942 is two to one. This is particularly advantageous when the control loops under consideration are parts of a hierarchical control loop structure. In such hierarchical control loop structures, the ratios of the early-late spacings of individual control loops are typically multiples of powers of two. In this case, it is very particularly preferable for the correction rule to be applied to the innermost control loops of the hierarchical control loop structure, when using a hierarchical control loop. The innermost control loops are in this case those whose respective delay locked loops DLL have the smallest early-late spacings.
It is very particularly preferable for at least either the first early-late spacing D1 or the second early-late spacing D2 to be considerably less than the expected minimum signal delay time difference between a plurality of propagation paths. This makes it possible to ensure that the correction of the control error can be carried out precisely as far as a specific resolution.
A preferred arrangement for demodulating a spread-spectrum signal which is received from a transmitter via a plurality of propagation paths has a generator for producing a local spread sequence and has a first control loop in which the received spread sequence is correlated with the local spread sequence on a first phase-angle pair which has a first early-late spacing 2xcex941. The arrangement furthermore has a second control loop in which the received spread sequence is correlated with the local spread sequence on a second phase-angle pair which has a second early-late spacing 2xcex942. The first and the second control loop emit a first point phase value T1 and a second point phase value T2, respectively, when in the steady state. The arrangement is characterized by a device for determining the phase value T0 (which is supplied to a demodulation correlator) of the local spread sequence with respect to the received spread sequence using the formula.
T0=(xcex942T1xe2x88x922xcex941T2)/(xcex942xe2x88x92xcex941) where D2 is chosen to be less than xcex941. The arrangement is particularly advantageously suitable for carrying out the method according to the invention.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and arrangement for minimizing the autocorrelation error in the demodulation of a spread-spectrum signal subject to multipath propagation, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.